In producing a multi-layered board, conductive patterns are formed on one surface of a resin sheet, serving as a base substrate of the multi-layered board, and via holes are formed on the other surface of the resin sheet so that the bottoms of the via holes should be the conductive patterns, and the via holes are filled with conductive paste. A plurality of resin sheets, in which the conductive patterns have been formed and the via holes have been filled with the conductive paste, are laminated and bonded together, so that one multi-layered board is formed. In these processes, inside the multi-layered board, each interlayer connection between adjoining conductive pattern layers is performed through the use of the conductive paste.
In such conductive paste filling processes, when the via holes are filled with the conductive paste, the conductive paste tends to adhered to a part of the resin sheet other than the via holes. Moreover, the resin sheet is likely to be damaged in such conductive paste filling operations. To prevent these troubles, a protective film is laminated on a surface of each resin sheet. This surface is on a side from which the conductive paste is filled into the via holes.
The via holes are formed, for example, by means of laser irradiation into the protective film and the resin sheet after the protective film is laminated on the resin sheet. Accordingly, penetration holes, the positions of which correspond to those of the via holes of the resin sheet, are formed in the protective film, and so the conductive paste is filled in the via holes of the resin sheet through the penetration holes.
In that process, when the via hole has a dust therein, or when the via hole is not formed to have a diameter in a predetermined size, the interlayer connections among the conductive patterns are likely to be performed sufficiently. Moreover, the via holes are poured with the conductive paste. In case the poured amount of the conductive paste is not sufficient, the interlayer connections are likely to become defective.
Accordingly, after the via holes are formed, or the conductive paste is poured, it is desired that the states of the via holes and filled degrees of the conductive paste are inspected by means of image recognition technology so as to reliably perform the interlayer connections.
However, a resin film, for example made of polyethylene terephthalate, is employed as the protective film so as to be easily laminated and detached. Such a resin film is transparent or white and therefore has high light reflectivity. Accordingly, contrast between the resin film and the conductive patterns on the bottom of the via holes, or that between the resin film and the conductive paste filling the via holes, is lowered. Therefore, it is difficult to recognize the shapes of the via holes, dusts therein, inadequate fillings of the conductive paste thereinto, and so on by means of the image recognition technology.